Transportation principles of liquid or dry bulk materials and of general freight heretofore have generally required separate types of vehicles for transporting each of these classes of materials. Basically, a van-type trailer vehicle and, in some cases, a flat bed trailer type vehicle are suitable for carrying most general freight loads. However, transport of liquid or dry bulk materials has heretofore required a trailer vehicle especially adapted for carrying these materials to the exclusion of general freight type loads. For example, vehicles adapted to carry dry bulk materials, such as grains, have included one or more permanently mounted rigid metallic container structures including hoppers or similar structures at the bottom thereof for unloading. Similarly, trailers for transporting liquid bulk materials have included similar permanent rigid containers having slope pans connected thereunder for unloading or, alternatively, tank-type trailers.
Thus, an operator wishing to carry both general freight and dry or liquid bulk materials must maintain a considerable number of trailers of appropriate design to accommodate each of these different types of loads. Further, in cases where one type of load is carried to a destination and it is desired to carry a different type of load on the return trip, a second vehicle must be brought in for the return trip, each vehicle making one-half of the journey empty. Obviously, this results in an inefficient use of vehicles and results in needless expenditure of time and effort as well as the expense inherent in expensive equipment being operated empty. Also, the types of vehicles heretofore utilized for carrying bulk materials including containers, bulkheads and the like of heavy rigid metallic construction, require heavy, expensive suspension systems and are relatively heavy vehicles. Thus, the total amount of bulk commodity transportable on such a vehicle may be unduly limited by the vehicle weight which comprises a large proportion of the gross vehicle weight allowed by law and/or regulation.
Further, with present bulk materials carrying vehicles, it is necessary to provide facilities for cleaning the containers prior to taking on new loads to prevent contamination from prior loads, especially when different materials are to be loaded in successive trips.